1. Field of the Invention
The present invention relates generally to fluid reaction surfaces, and more specifically to internally cooled airfoils used in a gas turbine engine.
2. Description of the Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98
A gas turbine engine, such as an aero engine or an industrial turbine engine, use turbine blades to convert energy from a hot gas stream into mechanical energy through rotation of a shaft connected to the blades. Because the efficiency of the engine increases with an increase in the hot gas stream temperature, higher efficiencies are obtained by providing internal cooling for the turbine airfoils such as the stationary vanes or the rotating blades.
A typical turbine blade will include a somewhat complex arrangement of cooling passages within the blade to maximize the cooling effect of the cooling air passing through the blade. A 3-pass serpentine flow circuit is a typical prior art cooling flow circuit used in turbine blades because it increases the cooling effectiveness by extending the length of the coolant flow path. In this circuit, a first leg supplies cooling air to a leading edge cooling channel which provides cooling air to a shower head arrangement for cooling the leading edge of the blade. Cooling air from the first leg that does not flow into the leading edge cooling channel passes into the second and third legs of the serpentine flow path, and then onward toward the trailing edge cooling holes that discharge out from the blade to cool the trailing edge.
The leading edge of the airfoil is exposed to the highest temperature and pressure acting on the airfoil. The forward region of the airfoil pressure surface is exposed to pressure from the hot gas that is slightly lower in pressure than is exposed on the leading edge. Because of this pressure difference, the cooling air flowing in the leading edge portion of the blade must be at a high enough pressure to provide back flow margin (BFM) in which the hot gas has a higher pressure than the leading edge cooling air and flows into the blade. U.S. Pat. No. 4,257,737 issued to Andress et al on Mar. 24, 1981 and entitled COOLED ROTOR BLADE shows this type of cooling arrangement. Cooling air enters a serpentine passageway at the blade root and flows into a first leg. A portion of the cooling air in the first leg flows through metering holes (28 in FIG. 1 of this patent) and into recessed portions (14 in FIG. 1 of this patent) before discharging through cooling holes 30 to cool the leading edge of the blade. Cooling air not diverted from the first leg continues on into the second and third legs of the serpentine flow circuit, and is discharged through cooling holes out the trailing edge of the blade. In the Andress et al prior art invention, the cooling air pressure in the first leg must be high enough to prevent backflow of the hot gas stream acting on the trailing edge. Because the cooling air pressure in the first leg is high, and because the second and third legs are in direct fluid communication with the first leg, most of the cooling air entering the first leg will flow into the second leg and this high pressure. More cooling air flows toward the trailing edge than is needed to adequately cool the trailing edge side of the blade. Therefore, in the Andress et al invention, an excess amount of cooling air is used in order to provide adequate leading edge cooling of the blade.
The prior art patent U.S. Pat. No. 6,491,496 issued to Starkweather on Dec. 10, 2002 and entitled TURBINE AIRFOIL WITH METERING PLATES FOR REFRESHER HOLES advances the cooling flow circuit of the above Andress et al patent by providing two serpentine cooling circuit, one for the leading edge and a second for the trailing edge. However, in the Starkweather patent, the pressure of the cooling air entering the leading edge serpentine circuit is the same as the pressure entering the trailing edge serpentine circuit. Because less cooling air flow is needed to cool the trailing edge, and because both pressures are the same, more cooling air flows into the trailing edge circuit than is needed.
Prior Art patent U.S. Pat. No. 5,403,156 issued to Arness et al on Apr. 4, 1995 and entitled INTEGRAL METER PLATE FOR TURBINE BLADE AND METHOD shows a turbine blade with a meter plate formed from a depending member extending from the root of the turbine blade just short of the edge of the bottom of the broach in the disk in the live rim area is cast integrally with the blade and serves to meter coolant flow from the on board injector to internally of the blade. The final dimension can be configured in the machining operation of the blade to attain the desired amount of coolant flow for internal blade cooling. In this patent, the metering device is not located in the serpentine flow circuit of the blade. Also, the metering device in this patent cannot be used to control the air flow volume to the leading edge portion of the blade. The metering device cannot be used to regulate the serpentine flow cooling flow circuit back flow margin (BFM) for the leading edge showerhead region and the out flow margin (OFM) for the second leg of the down-pass as well as the cooling flow pressure and flow rate to the trailing edge.
Another prior art patent, U.S. Pat. No. 6,186,741 B1 issued to Webb et al on Feb. 13, 2001 and entitled AIRFOIL COMPONENT HAVING INTERNAL COOLING AND METHOD OF COOLING, shows a turbine blade having a leading edge and a trailing edge, shows a turbine blade with a serpentine cooling flow circuit within the blade. The blade includes a leading edge cooling passageway (128 in this patent), a trailing edge cooling passageway (132 in this patent), and a serpentine cooling circuit between the LE and TE passageways having a 5-pass circuit. A core tie hole fluidly connected adjacent cooling passageways such that cooling air flowing into the leading edge cooling passageway can flow into the trailing edge cooling passageway. This patent also includes a metering plate having a metering hole therein located at the entrance to the trailing edge cooling passageway for the purpose of lowering the pressure of the cooling air in the trailing edge cooling passageway to the point where the pressure in the adjacent 5th leg of the serpentine cooling circuit is about equal. In the Webb et al patent, one problem is that the core tie holes cannot be changed after the blade has been cast, and therefore the cooling flow through the various circuits cannot be adjusted depending upon the operating conditions of the blade.
It is therefore an object of the present invention to provide for an improved serpentine circuit cooling flow through a turbine airfoil such that adequate cooling air pressure to the hot region of the airfoil is obtained and without over-pressuring the trailing edge region of the airfoil.
It is another object of the present invention to provide for a turbine airfoil with a metering device that can be formed into the airfoil during casting, and that can be changed easily before installation into the turbine.